# Name

MochiKit.Base - functional programming and useful comparisons

# Synopsis

    myObjectRepr = function () {
        // gives a nice, stable string representation for objects,
        // ignoring any methods
        var keyValuePairs = [];
        for (var k in this) {
            var v = this[k];
            if (typeof(v) != 'function') {
                keyValuePairs.push([k, v]);
            }
        };
        keyValuePairs.sort(compare);
        return "{" + map(
            function (pair) {
                return map(repr, pair).join(":");
            },
            keyValuePairs
        ).join(", ") + "}";
    };

    // repr() will look for objects that have a repr method
    myObjectArray = [
        {"a": 3, "b": 2, "repr": myObjectRepr},
        {"a": 1, "b": 2, "repr", myObjectRepr}
    ];

    // sort it by the "a" property, check to see if it matches
    myObjectArray.sort(keyComparator("a"));
    expectedRepr = '[{"a": 1, "b": 2}, {"a": 3, "b": 2}]';
    assert( repr(myObjectArray) == expectedRepr );

    // get just the "a" values out into an array
    sortedAValues = map(itemgetter("a"), myObjectArray);
    assert( compare(sortedAValues, [1, 3]) == 0 );

    // serialize an array as JSON, unserialize it, expect something equivalent
    myArray = [1, 2, "3", null, undefined];
    assert( objEqual(evalJSON(serializeJSON(myArray)), myArray) );

# Description

MochiKit.Base is the foundation for the MochiKit suite. It provides:

- An extensible comparison facility (`compare`, `registerComparator`)
- An extensible programmer representation facility (`repr`, `registerRepr`)
- An extensible JSON [1] serialization and evaluation facility (`serializeJSON`, `evalJSON`, `registerJSON`)
- A simple adaptation facility (`AdapterRegistry`)
- Convenience functions for manipulating objects (`update`, `extend`, etc.)
- Array-based functional programming (`map`, `filter`, `setdefault`, etc.)
- Bound and partially applied functions (`bind`, `partial`)

Python users will feel at home with MochiKit.Base, as the facilities are quite similar to those available as part of Python and the Python standard library.

# Dependencies

None.

# Overview

## Comparison

The comparators (operators for comparison) in JavaScript are deeply broken, and it is not possible to teach them new tricks.

MochiKit exposes an extensible comparison facility as a simple compare(a, b) function, which should be used in lieu of JavaScript's operators whenever comparing objects other than numbers or strings (though you can certainly use compare for those, too!).

The compare function has the same signature and return value as a sort function for `Array.prototype.sort`, and is often used in that context.

Defining new comparators for the compare function to use is done by adding an entry to its AdapterRegistry with the registerComparator function.

## Programmer Representation

JavaScript's default representation mechanism, `toString`, is notorious for having terrible default behavior. It's also very unwise to change that default, as other JavaScript code you may be using may depend on it.

It's also useful to separate the concept of a "string representation" and a "string representation for programmers", much like Python does with its str and repr protocols.

repr provides this programmer representation for JavaScript, in a way that doesn't require object prototype hacking: using an AdapterRegistry.

Objects that implement the repr protocol can either implement a `.repr()` or `.__repr__()` method, or they can simply have an adapter setup to generate programmer representations. By default, the registry provides nice representations for `null`, `undefined`, `Array`, and objects or functions with a `NAME` attribute that use the default `toString`. For objects that `repr` doesn't already understand, it simply defaults to `toString`, so it will integrate seamlessly with code that implements the idiomatic JavaScript `toString` method!

To define a programmer representation for your own objects, simply add a `.repr()` or `.__repr__()` method that returns a string. For objects that you didn't create (e.g., from a script you didn't write, or a built-in object), it is instead recommended that you create an adapter with registerRepr.

## JSON Serialization

JSON [1], JavaScript Object Notation, is a widely used serialization format in the context of web development. It's extremely simple, lightweight, and fast. In its essence, JSON is a restricted subset of JavaScript syntax suitable for sending over the wire that can be unserialized with a simple eval. It's often used as an alternative to XML in "AJAX" contexts because it is compact, fast, and much simpler to use for most purposes.

To create a JSON serialization of any object, simply call serializeJSON() with that object. To unserialize a JSON string, simply call evalJSON() with the serialization.

In order of precedence, serializeJSON coerces the given argument into a JSON serialization:

1. Primitive types are returned as their JSON representation: `undefined`, `string`, `number`, `boolean`, `null`.
2. If the object has a `__json__` or `json` method, then it is called with no arguments. If the result of this method is not the object itself, then the new object goes through rule processing again (e.g. it may return a string, which is then serialized in JSON format).
3. If the object is array-like (has a length property that is a number, and is not a function), then it is serialized as a JSON array. Each element will be processed according to these rules in order. Elements that can not be serialized (e.g. functions) will be replaced with `undefined`.
4. The `jsonRegistry` AdapterRegistry is consulted for an adapter for this object. `JSON` adapters take one argument (the object), and are expected to behave like a `__json__` or `json` method (return another object to be serialized, or itself).
5. If no adapter is available, the object is enumerated and serialized as a JSON object (name:value pairs). All names are expected to be strings. Each value is serialized according to these rules, and if it can not be serialized (e.g. methods), then that name:value pair will be skipped.

## Adapter Registries

MochiKit makes extensive use of adapter registries, which enable you to implement object-specific behaviors for objects that you do not necessarily want to modify, such as built-in objects. This is especially useful because JavaScript does not provide a method for hiding user-defined properties from `for propName in obj` enumeration.

AdapterRegistry is simply an encapsulation for an ordered list of "check" and "wrap" function pairs. Each AdapterRegistry instance should perform one function, but may have multiple ways to achieve that function based upon the arguments. One way to think of it is as a poor man's generic function, or multiple dispatch (on arbitrary functions, not just type!).

Check functions take one or more arguments, and return `true` if the argument list is suitable for the wrap function. Check functions should perform "cheap" checks of an object's type or contents, before the "expensive" wrap function is called.

Wrap functions take the same arguments as check functions and do some operation, such as creating a programmer representation or comparing both arguments.

## Convenience Functions

Much of :mochiref:MochiKit.Base is there to simply remove the grunt work of doing generic JavaScript programming.

Need to take every property from one object and set them on another? No problem, just call update(dest, src)! What if you just wanted to update keys that weren't already set? Look no further than setdefault(dest, src[, ...]).

Want to return a mutable object, but don't want to suffer the consequences if the user mutates it? Just clone(it) and you'll get a copy-on-write clone. Cheaper than a copy!

Need to extend an Array with another array? Or even an array-like object such as a `NodeList` or the special `arguments` object? Even if you need to skip the first few elements of the source array-like object, it's no problem with extend(dstArray, srcArrayLike[, skip])!

Wouldn't it be convenient to have all of the JavaScript operators were available as functions somewhere? That's what the operators table is for, and it even comes with additional operators based on the compare function.

Need to walk some tree of objects and manipulate or find something in it? A DOM element tree perhaps? Use nodeWalk(node, visitor)!

There's plenty more, so check out the API Reference below.

## Functional Programming

Functional programming constructs such as map and filter can save you a lot of time, because JavaScript iteration is error-prone and arduous. Writing less code is the best way to prevent bugs, and functional programming can help you do that.

MochiKit.Base ships with a few simple Array-based functional programming constructs, namely map and filter, and their "extended" brethren, xmap and xfilter.

map(func, arrayLike[, ...]) takes a function and an array-like object, and creates a new `Array`. The new `Array` is the result of `func(element)` for every element of `arrayLike`, much like the `Array.prototype.map` extension in Mozilla. However, MochiKit.Base takes that a step further and gives you the full blown Python version of map, which will take several array-like objects, and calls the function with one argument per given array-like, like this:

    var arrayOne = [1, 2, 3, 4, 5];
    var arrayTwo = [1, 5, 2, 4, 3];
    var arrayThree = [5, 2, 1, 3, 4];
    var biggestElements = map(objMax, arrayOne, arrayTwo, arrayThree);
    assert( objEqual(biggestElements, [5, 5, 3, 4, 5]) );

filter(func, arrayLike[, self]) takes a function and an array-like object, and returns a new `Array`. This is basically identical to the `Array.prototype.filter` extension in Mozilla. self, if given, will be used as `this` in the context of func when called.

xmap and xfilter are just special forms of map and filter that accept a function as the first argument, and use the extra arguments as the array-like. Not terribly interesting, but a definite time-saver in some cases.

If you appreciate the functional programming facilities here, you should definitely check out MochiKit.Iter, which provides full blown iterators, MochiKit.Iter.range, MochiKit.Iter.reduce, and a near-complete port of Python's itertools [2] module, with some extra stuff thrown in for good measure!

## Bound and Partial Functions

JavaScript's method-calling special form and lack of bound functions (functions that know what `this` should be) are one of the first stumbling blocks that programmers new to JavaScript face. The bind(func, self) method fixes that right up by returning a new function that calls func with the right `this`.

In order to take real advantage of all this fancy functional programming stuff, you're probably going to want partial application. This allows you to create a new function from an existing function that remembers some of the arguments. For example, if you wanted to compare a given object to a slew of other objects, you could do something like this:

    compareWithOne = partial(compare, 1);
    results = map(compareWithOne, [0, 1, 2, 3]);
    assert( objEqual(results, [-1, 0, 1, 1]) );

One of the better uses of partial functions is in MochiKit.DOM, which is certainly a must-see for those of you creating lots of DOM elements with JavaScript!

# API Reference

## Errors

NotFound:

> A singleton error raised when no suitable adapter is found

## Constructors

AdapterRegistry:

> A registry to facilitate adaptation.

> All check/wrap functions in this registry should be of the same arity.

AdapterRegistry.prototype.register(name, check, wrap[, override]):

> The check function should return true if the given arguments are appropriate for the wrap function.

> If override is given and true, the check function will be given highest priority. Otherwise, it will be the lowest priority adapter.

AdapterRegistry.prototype.match(obj[, ...]):

> Find an adapter for the given arguments.

> If no suitable adapter is found, throws NotFound.

AdapterRegistry.prototype.unregister(name):

> Remove a named adapter from the registry

NamedError:

> Convenience constructor for creating new errors (e.g. NotFound)

## Functions

arrayEqual(self, arr):

> Compare two arrays for equality, with a fast-path for length differences.

bind(func, self[, arg, ...]):

> Return a copy of `func` bound to `self`. This means whenever and however the returned function is called, `this` will always reference the given `self`.

> Calling bind(func, self) on an already bound function will return a new function that is bound to the new `self`! If `self` is `undefined`, then the previous `self` is used. If `self` is `null`, then the `this` object is used (which may or may not be the global object). To force binding to the global object, you should pass it explicitly.

> Additional arguments, if given, will be partially applied to the function. These three expressions are equivalent and return equally efficient functions (bind and partial share the same code path):

> - bind(oldfunc, self, arg1, arg2)
> - bind(partial(oldfunc, arg1, arg2), self)
> - partial(bind(oldfunc, self), arg1, arg2)

bindMethods(self):

> Bind all methods of `self` present on self to `self`, which gives you a semi-Pythonic sort of instance.

clone(obj):

> Return a new object using `obj` as its prototype. Use this if you want to return a mutable object (e.g. instance state), but don't want the user to mutate it. If they do, it won't have any effect on the original `obj`.

> Note that this is a shallow clone, so mutable properties will have to be cloned separately if you want to "protect" them.

compare(a, b):

> Compare two objects in a sensible manner. Currently this is:

> > 1. `undefined` and `null` compare equal to each other
> > 2. `undefined` and `null` are less than anything else
> > 3. If JavaScript says `a == b`, then we trust it
> > 4. comparators registered with registerComparator are used to find a good comparator. Built-in comparators are currently available for `Array`-like and `Date`-like objects.
> > 5. Otherwise hope that the built-in comparison operators do something useful, which should work for numbers and strings.
> > 6. If neither `a < b` or `a > b`, then throw a `TypeError`

> Returns what one would expect from a comparison function:

> Value | Condition
> 0 | a == b
> 1 | a > b
> -1 | a < b

concat(lst[, ...]):

> Concatenates all given array-like arguments and returns a new `Array`:

>     var lst = concat(["1","3","5"], ["2","4","6"]);
>     assert( lst.toString() == "1,3,5,2,4,6" );

counter(n=1):

> Returns a function that will return a number one greater than the previous returned value, starting at `n`. For example:

>     nextId = counter()
>     assert( nextId() == 1 )
>     assert( nextId() == 2 )

> For an iterator with this behavior, see MochiKit.Iter.count.

extend(self, obj[, skip]):

> Mutate an array by extending it with an array-like obj, starting with the "skip" index of obj. If null is given as the initial array, a new one will be created.

> This mutates and returns the given array, be warned.

evalJSON(aJSONString):

> Unserialize a JSON [1] represenation of an object.

> Note that this uses the `eval` function of the interpreter, and therefore trusts the contents of `aJSONString` to be safe. This is acceptable when the JSON and JavaScript application originate from the same server, but in other scenarios it may not be the appropriate security model. Currently, a validating JSON parser is beyond the scope of MochiKit, but there is one available from json.org [1].

filter(fn, lst):

> Returns a new `Array` composed of all elements from `lst` where `fn(lst[i])` returns a true value.

> If `fn` is `null`, `operator.truth` will be used.

forward(name):

> Returns a function that forwards a method call to `this.name(...)`

isArrayLike(obj[, ...]):

> Returns `true` if all given arguments are `Array`-like (have a `.length` property and `typeof(obj) == 'object'`)

isDateLike(obj[, ...]):

> Returns `true` if all given arguments are `Date`-like (have a `.getTime()` method)

isNotEmpty(obj[, ...]):

> Returns `true` if all the given `Array`-like or string arguments are not empty `(obj.length > 0)`

isNull(obj[, ...]):

> Returns `true` if all arguments are `null`.

isUndefinedOrNull(obj[, ...]):

> Returns `true` if all arguments are undefined or `null`

itemgetter(name):

> Returns a `function(obj)` that returns `obj[name]`

items(obj):

> Return an `Array` of `[propertyName, propertyValue]` pairs for the given `obj` (in the order determined by `for propName in obj`).

keyComparator(key[, ...]):

> A comparator factory that compares `a[key]` with `b[key]`. e.g.:

>     var lst = ["a", "bbb", "cc"];
>     lst.sort(keyComparator("length"));
>     assert( lst.toString() == "a,cc,bbb" );

keys(obj):

> Return an `Array` of the property names of an object (in the order determined by `for propName in obj`).

listMax(lst):

> Return the largest element of an `Array`-like object, as determined by compare. This is a special form of listMinMax, specifically partial(listMinMax, 1).

listMin(lst):

> Return the smallest element of an `Array`-like object, as determined by compare. This is a special form of listMinMax, specifically partial(listMinMax, -1).

listMinMax(which, lst):

> If `which == -1` then it will return the smallest element of the `Array`-like `lst`. This is also available as listMin(lst).

> If `which == 1` then it will return the largest element of the array-like lst. This is also available as listMax(list).

map(fn, lst[, ...]):

> Return a new array composed of the results of `fn(x)` for every `x` in `lst`.

> If fn is `null`, and only one sequence argument is given the identity function is used.

> > map(null, lst) -> `lst.slice()`;

> If `fn` is `null`, and more than one sequence is given as arguments, then the `Array` function is used, making it equivalent to zip.

> > map(null, p, q, ...)
> >   -> zip(p, q, ...) -> `[[p0, q0, ...], [p1, q1, ...], ...];`

merge(obj[, ...]):

> Create a new instance of `Object` that contains every property from all given objects. If a property is defined on more than one of the objects, the last property is used.

> This is a special form of update(self, obj[, ...]), specifically, it is defined as partial(update, null).

nameFunctions(namespace):

> Given a namespace with a `NAME` property, find all functions in it and give them nice `NAME` properties too (for use with repr). e.g.:

>     namespace = {
>         NAME: "Awesome",
>         Dude: function () {}
>     }
>     nameFunctions(namespace);
>     assert( namespace.Dude.NAME == 'Awesome.Dude' );

objEqual(a, b):

> Compare the equality of two objects.

nodeWalk(node, visitor):

> Non-recursive generic node walking function (e.g. for a DOM)

> `node`:
>   The initial node to be searched.
> `visitor`:
>   The visitor function, will be called as `visitor(node)`, and should return an `Array`-like of nodes to be searched next (e.g. `node.childNodes`).

objMax(obj[, ...]):

> Return the maximum object out of the given arguments. This is similar to listMax, except is uses the arguments instead of a given `Array`-like.

objMin(obj[, ...]):

> Return the minimum object out of the given arguments. This is similar to listMin, except it uses the arguments instead of a given `Array`-like.

operator:

> A table of JavaScript's operators for usage with map, filter, etc.

> Unary Logic Operators:

> Operator | Implementation | Description
> --- | --- | ---
> truth(a) | !!a | Logical truth
> lognot(a) | !a | Logical not
> identity(a) | a | Logical identity

> Unary Math Operators:

> Operator | Implementation | Description
> --- | --- | ---
> not(a) | ~a | Bitwise not
> neg(a) | -a | Negation

> Binary Operators:

> Operator | Implementation | Description
> --- | --- | ---
> add(a, b) | a + b | Addition
> sub(a, b) | a - b | Subtraction
> div(a, b) | a / b | Division
> mod(a, b) | a % b | Modulus
> and(a, b) | a & b | Bitwise and
> or(a, b) | a | b | Bitwise or
> xor(a, b) | a ^ b | Bitwise exclusive or
> lshift(a, b) | a << b | Bitwise left shift
> rshift(a, b) | a >> b | Bitwise signed right shift
> zrshfit(a, b) | a >>> b | Bitwise unsigned right shift

> Built-in Comparators:

> Operator | Implementation | Description
> --- | --- | ---
> eq(a, b) | a == b | Equals
> ne(a, b) | a != b | Not equals
> gt(a, b) | a > b | Greater than
> ge(a, b) | a >= b | Greater than or equal to
> lt(a, b) | a < b | Less than
> le(a, b) | a <= b | Less than or equal to

> Extended Comparators (uses compare):

> Operator | Implementation | Description
> --- | --- | ---
> ceq(a, b) | compare(a, b) == 0 | Equals
> cne(a, b) | compare(a, b) != 0 | Not equals
> cgt(a, b) | compare(a, b) == 1 | Greater than
> cge(a, b) | compare(a, b) != -1 | Greater than or equal to
> clt(a, b) | compare(a, b) == -1 | Less than
> cle(a, b) | compare(a, b) != 1 | Less than or equal to

> Binary Logical Operators:

> Operator | Implementation | Description
> --- | --- | ---
> logand(a, b) | a && b | Logical and
> logor(a, b) | a || b | Logical or
> contains(a, b) | b in a | Has property (note order)

parseQueryString(encodedString[, useArrays=false]):

> Parse a name=value pair URL query string into an object with a property for each pair. e.g.:

>     var args = parseQueryString("foo=value%20one&bar=two");
>     assert( args.foo == "value one" && args.bar == "two" );

> If you expect that the query string will reuse the same name, then give `true` as a second argument, which will use arrays to store the values. e.g.:

>     var args = parseQueryString("foo=one&foo=two");
>     assert( args.foo[0] == "one" && args.foo[1] == "two" );

partial(func, arg[, ...]):

> Return a partially applied function, e.g.:

>     addNumbers = function (a, b) {
>         return a + b;
>     }

>     addOne = partial(addNumbers, 1);

>     assert(addOne(2) == 3);

> partial is a special form of bind that does not alter the bound `self` (if any). It is equivalent to calling:

>     bind(func, undefined, arg[, ...]);

> See the documentation for bind for more details about this facility.

Note

This could be used to implement, but is NOT currying.

queryString(names, values):

> Creates a URL query string from a pair of array-like objects representing `names` and `values`. Each name=value pair will be URL encoded by urlEncode. name=value pairs with a value of `undefined` or `null` will be skipped. e.g.:

>     var keys = ["foo", "bar"];
>     var values = ["value one", "two"];
>     assert( queryString(keys, values) == "foo=value%20one&bar=two" );

> Alternate form 1:
>   queryString(domElement)

> If MochiKit.DOM is loaded, one argument is given, and that argument is either a string or has a `nodeType` property greater than zero, then `names` and `values` will be the result of MochiKit.DOM.formContents(domElement).

> Alternate form 2:
>   queryString({name: value, ...})

> Note that when using the alternate form, the order of the name=value pairs in the resultant query string is dependent on how the particular JavaScript implementation handles `for (..in..)` property enumeration.

> When using the second alternate form, name=value pairs with `typeof(value) == "function"` are ignored. This is a workaround for the case where a poorly designed library has modified `Object.prototype` and inserted "convenience functions".

registerComparator(name, check, comparator[, override]):

> Register a comparator for use with compare.

> `name` should be a unique identifier describing the comparator.

> `check` is a `function(a, b)` that returns `true` if `a` and `b` can be compared with `comparator`.

> `comparator` is a `function(a, b)` that returns:

> Value | Condition
> 0 | a == b
> 1 | a > b
> -1 | a < b

> `comparator` is guaranteed to only be called if `check(a, b)` returns a `true` value.

> If `override` is `true`, then it will be made the highest precedence comparator. Otherwise, the lowest.

registerJSON(name, check, simplifier[, override]):

> Register a simplifier function for use with serializeJSON.

> `name` should be a unique identifier describing the serialization.

> `check` is a `function(obj)` that returns `true` if `obj` can can be simplified for serialization by `simplifier`.

> `simplifier` is a `function(obj)` that returns a simpler object that can be further serialized by serializeJSON. For example, you could simplify Date-like objects to ISO 8601 timestamp strings with the following simplifier:

>     var simplifyDateAsISO = function (obj) {
>         return toISOTimestamp(obj, true);
>     };
>     registerJSON("DateLike", isDateLike, simplifyDateAsISO);

> `simplifier` is guaranteed to only be called if `check(obj)` returns a `true` value.

> If `override` is `true`, then it will be made the highest precedence comparator. Otherwise, the lowest.

registerRepr(name, check, wrap[, override]):

> Register a programmer representation function. repr functions should take one argument and return a string representation of it suitable for developers, primarily used when debugging.

> If `override` is given, it is used as the highest priority repr, otherwise it will be used as the lowest.

repr(o):

> Return a programmer representation for an object. See the Programmer Representation overview for more information about this function.

reverseKeyComparator(key):

> A comparator factory that compares `a[key]` with `b[key]` in reverse. e.g.:

>     var lst = ["a", "bbb", "cc"];
>     lst.sort(reverseKeyComparator("length"));
>     assert(lst.toString() == "bbb,cc,aa");

serializeJSON(anObject):

> Serialize any object in the JSON [1] format, see JSON Serialization for the coercion rules. For unserializable objects (functions that do not have an adapter, `__json__` method, or `json` method), this will return `undefined`.

> For those familiar with Python, JSON is similar in scope to pickle, but it can not handle recursive object graphs.

setdefault(self, obj[, ...]):

> Mutate an object by adding all properties from other object(s) that it does not already have set.

> If `self` is `null`, a new `Object` instance will be created and returned.

> This mutates and returns the given `self`, be warned.

typeMatcher(typ[, ...]):

> Given a set of types (as string arguments), returns a `function(obj[, ...])` that will return `true` if the types of the given arguments are all members of that set.

update(self, obj[, ...]):

> Mutate an object by replacing its key:value pairs with those from other object(s). Key:value pairs from later objects will overwrite those from earlier objects.

> If null is given as the initial object, a new one will be created.

> This mutates and returns the given object, be warned.

> A version of this function that creates a new object is available as merge(a, b[, ...])

updatetree(self, obj[, ...]):

> Mutate an object by replacing its key:value pairs with those from other object(s). If a given key has an object value in both `self` and `obj`, then this function will be called recursively, updating instead of replacing that object.

> If null is given as the initial object, a new one will be created.

> This mutates and returns the given object, be warned.

urlEncode(unencoded):

> Converts a string into a URL-encoded string. Note that, in this implementation, spaces are converted to %20 instead of "+". e.g.:

>     assert( URLencode("1+2=2") == "1%2B2%3D2");

xfilter(fn, obj[, ...]):

> Returns a new `Array` composed of the arguments where `fn(obj)` returns a true value.

> If `fn` is `null`, `operator.truth` will be used.

xmap(fn, obj[, ...):

> Return a new `Array` composed of `fn(obj)` for every `obj` given as an argument.

> If `fn` is `null`, `operator.identity` is used.

# See Also

[1] | (1, 2, 3, 4, 5) JSON, JavaScript Object Notation: http://json.org/

[2] | Python's itertools module: http://docs.python.org/lib/module-itertools.html

# Authors

- Bob Ippolito <bob@redivi.com>

# Copyright

Copyright 2005 Bob Ippolito <bob@redivi.com>. This program is dual-licensed free software; you can redistribute it and/or modify it under the terms of the MIT License or the Academic Free License v2.1.